Three sisters were found stabbed to death in the Gaza Strip on Sunday, raising suspicion they were killed by relatives because of suspected immoral behavior, a human rights organization said. The three sisters, 16-year-old Nahed Hija and her sisters, 19-year-old Suha and 22-year-old Lina, were found dead from multiple stab wounds, buried in a shallow grave in the central Gaza Strip early Sunday morning, said Hamdi Shakkour of the Palestinian Center for Human Rights. Shakkour said they suspected the women were victims of "honor crimes," in which women are murdered by male relatives because of suspected intimate relations - not necessarily sex - outside of marriage. The Hamas force that polices the Gaza Strip said in a statement it was investigating the deaths. Honor killings are practiced throughout the Arab world. At least 12 women were killed for honor crimes in the West Bank and Gaza Strip last year, but this is the first time three sisters were murdered together. In February, three unrelated woman were killed within a 24-hour period in the Gaza Strip. At the time, there were concerns that vigilante moral groups were behind the killings. The Palestinian judiciary does not take "honor crimes" seriously, Shakkour said. Perpetrators of "honor crimes" are often given light sentences of a few years, while others convicted of murder under other circumstances are sentenced to death.